energy
energy energy. it's all about energy. to be useful energy needs four things: 1) quantity. 6 billion people use a lot. 2) high return. it's called eroei = energy returned on energy invested. or something similar. basically means you want to do very little work to get a whole lot energy. 3) low pollution. this is kinda redundant. it can be absorbed into the return equation. ie you have to spend some of the harvested energy cleaning up any mess you made acquiring the energy or using it. and 4) density. we need to store lots of energy per pound of fuel and storage device. oil is really cool because 1) there's lots of it. like a trillion barrels. 2) eroei is 50. 3) pollution's not perfect, but manageable. 4) it's really dense. 1 pound of gasoline has as much energy as 100 pounds of batteries.